The storage of electricity has become a major issue of stability of the electricity grids in the short and medium terms. Historically, the electricity grids were designed to be “downstream” that is to say to go from big power plants to the consumers. Everything was therefore dimensioned (production and distribution) according to this unidirectional pattern and with a control of the productions to match as closely as possible the demand for electricity.
In the last few years, due to the rarefaction of fossil fuels on the one hand and to their harmful effects on the environment on the other hand, new electricity production means have been introduced in the energy mix, based primarily on renewable energies.
But one of the characteristics of these new production tools implemented heavily is their variability, even their intermittence. Certainly these energies are predictable in the short and medium terms, but they are irregular, and contrary to the historical production means, they cannot be controlled. Furthermore, as yet totally devoid of any storage means, these new tools must feed their electricity into the distribution grid according to their production, in real time.
The result of this variability is the need to make use of the storage means (mainly hydraulic) and of some power plants whose production is highly reactive, such as oil or gas-fired power plants, to ensure at all times the balance between supply and demand for electricity. But this exercise has its limitations because the hydraulic storage reached saturation and flame production tools have non-zero startup times and costs, leading the power plant operators to subsidize operators to consume the electricity rather than having to stop these power plants and to restart them shortly after. This operation meets a physical need for preservation of grids which, without this operation, would see the voltage and/or frequency of their electricity fluctuate outside the tolerances.
It is to avoid this type of operations that all the energy players have started looking for new electricity storage tools which can be mobilized rapidly and on a large scale.
Many techniques are contemplated such as the improvement of the hydraulic storage, the storage by compressed air, the electrochemical storage, by hydrogen storage or by flywheel.
However, these various techniques are limited by a lack of operating sites (hydraulic) or by problems of still too high costs (electrochemistry, hydrogen or flywheel) and again for many years.
There is also the hardly used way of storing electricity thermodynamically, field in which many innovations are in preparation, by coupling refrigeration cycles and organic Rankine cycles (ORC for short).
However, these new systems and methods have many drawbacks.
For instance, the hot and cold sources are made of non-constant storage temperature materials. Moreover, these systems have many exchanger stages, which is detrimental to efficiencies, too low to contemplate an industrial exploitation.
To improve efficiencies, it is useful to use hot sources and cold sources with constant temperatures and heat exchangers immersed in these sources.
This is particularly the case of the U.S. Pat. No. 8,484,986, relating to electricity storage systems by thermodynamic means, comprising a heat exchange circuit between a cold source and a hot source with constant temperatures, allowing in a charge cycle to store electrical energy and in a discharge cycle to generate electrical energy.